1. Field of the Invention
The present invention relates to a method of manufacturing an optical element, particularly, to a method of manufacturing a glass optical element having an optically functional surface produced directly by press-molding.
2. Related Background Art
Recently there has been developed the method of manufacturing an optical element, wherein glass material for molding of an optical element, for instance, glass blank pre-molded to certain shape is accommodated between the upper and the lower mold members having the predetermined surface precision and it is press-molded under heating, so that post-working steps such as cutting and grinding are unnecessary.
When an optical element is manufactured by such press-molding, the mold generally employed has such construction that the upper mold member and the lower mold member are set face to face slidably in the mold body and glass material for molding is introduced into the area surrounded by these members and the upper and the lower mold members are brought relatively close to each other to press it to the predetermined shape. At this time non-oxidizing atmosphere such as nitrogen atmosphere is employed to prevent oxidation of the mold members and said glass material is heated to the temperature suitable for molding, for example, to the temperature where the glass material for molding has viscosity of 10.sup.8 to 10.sup.12 poise, After the mold is closed, the glass material is pressed for predetermined period and the molding surface of the mold member is transferred to the surface of the material. Thereafter, said mold member is cooled down to glass transition temperature and pressing pressure is removed in such state. When molding is completed as aforesaid, the mold is opened and the product is taken out.
Alternatively, it may be so arranged that the glass material is heated to an appropriate temperature before introducing it into the mold member or the glass material is introduced into the mold member after heating them to the molding temperature. It may further be so arranged that in the process of transportation of glass material together with the mold member, the work for heating, pressing and cooling may be made continuous and at high speed.
The method of manufacturing an optical element as aforesaid and the apparatus used therefore are disclosed, for example, in the Japanese Patent Application Laid-open Nos. 58-84134, 49-97009, British Patent No. 378199, Japanese Patent Application Laid-open Nos. 63-11529, 59-150728 and 61-26528.
Here, the work for releasing product from the mold after press-molding is a problem. Immediately after pressing, the product is strongly ahered to the mold member. But, in the process in which the temperature decreases, thermal stress is produced due to the difference in thermal expansion coefficient between the mold member and the glass material, thereby releasing the product. The timing of such releasing the product is, however, not constant, and besides it is undecided whether the product is adhering to the upper mold, or the lower mold, when the mold is opened. Therefore, if the product is adhering, for example, to the upper mold, the product is released due to its own weight when the upper mold is elevated. As a result, the product falls on the lower mold, so that the product may be damaged or the molding surface of the lower mold may be damaged. When the product remains adhering to the upper mold, auto-handling is impossible in a subsequent step for taking out the product.
Such adhering of product to the upper mold is a factor to obstruct smooth operation of molding apparatus. In regard to cooling and releasing steps in the molding apparatus, there are already known Japanese Patent Application Laid-open Nos. 62-91430, 60-81032, 52-45613, 61-32263, etc. However, the manners for cooling and releasing disclosed in these patent publications are simple and no description is made for the solution of the problems related to adhering of product to the upper mold.